The invention relates to tube connectors and particularly to connectors of the type used for attaching flexible plastic tubing to devices such as pneumatic valves and cylinders. Most prior art connectors of this type take the form of special fittings which must be threaded into a correspondingly threaded hole in the device, thus requiring a considerable amount of space for access to the fitting by a wrench, for example. U.S. Pat. No. 4,021,062 discloses a coupling wherein a tube retaining collet element and O-ring are permanently mounted inside a fitting member. U.S. Pat. No. 4,059,295 discloses a coupling in which a relatively thick walled two-piece sleeve member surrounds the tubing and engages it with a series of internal ribs. The sleeve member is retained relative to the coupling body by a U-shaped clip and is spaced from an O-ring by a ring member. A somewhat similar coupling is disclosed on pages 3662, 3663 of IBM Technical Disclosure Bulletin, Vol. 20, No. 9 dated February 1978. The latter publication discloses a fitting in which a sleeve member is held in place inside a body member by a retaining ring which frictionally engages the inner wall of the body member. The tubing is removably inserted in the sleeve member and is retained to the sleeve, but not to the body member, by a O-shaped clip which passes through transverse slots on opposite sides of the sleeve. My own U.S. Pat. No. 4,453,747 shows a sleeve member which is positively retained relative to the body member as well as the tubing by a retaining clip having a bifurcated end portion. The body member must be provided with a pair of apertures positioned transverse to the axis of the tubing to accommodate the bifurcated end legs of the retaining clip.